This invention relates to a single crystal of semiconductive material grown on a substrate formed of a crystal of electrically insulating material and more particularly to such a single crystal having a reduced difference in lattice constant between the single crystal and the insulating crystal.
It is well known that, for example, with circuit elements formed in a two-dimensional pattern of a single crystal of semiconductive material as in integrated circuits, each of the circuit elements is electrically insulated from the other elements or its substrate by the associated pn junction. In such a case, stray capacitances and/or deteriorated insulation may frequently adversely affect the characteristics of the resulting semiconductor devices and more particularly the frequency response thereof or the noise therefrom. Recently, therefore, it is being widely practiced to use single crystals of silicon, for example, grown upon a substrate of electrically insulating materials such as sapphire (Al.sub.2 O.sub.3), spinel (MgO.Al.sub.2 O.sub.3 or MgAl.sub.2 O.sub.4) or quartz (SiO.sub.2). When a single crystal of semiconductive material has been grown on a substrate of electrically insulating material as above described, the substrate ensures that circuit elements disposed on the single crystal can be extremely easily electrically insulated from one another while any parasitic capacitance involved can be kept extremely low.
However the crystal growth on such an insulating substrate, known as the so-called heterojunction, involves a large misfit between the two crystal structures. The misfit is very large as compared with, for example, the growth of silicon on a silicon substrate or "Si on Si" due to a difference in lattice constant between the grown material and the material of the substrate. In other words, the layer grown on the substrate has therein defects which are due to strains developed in the layer during its growth. For example, silicon has a lattice constant of about 5.4302 A and is of a face centered cubic structure whereas sapphire, which is one of the electrically insulating materials widely used as a substrate, is of a rhombohedral structure having lattice constants of a=4.758 A and c=12.911 A. Also spinel, which is used for the same purpose, is of the face centered cubic structure and has a lattice constant of 8.088 A. Upon growing a single crystal of silicon on a substrate formed of such an insulating material, it is impossible to bond all silicon atoms with all atoms of insulating material and therefore a difference in lattice constant between the two materials per se does not indicate a measure of the mismatch. Although the mechanism by which dissimilar lattices are mismatched is complicated as compared with similar lattices, as in "Si on Si", it is said that the mismatch for dissimilar lattices is in the order of from 1 to 2% as a result of experiments or on the basis of certain assumptions. For "Si on Si" the doping of the silicon with an impurity exhibiting the effect of decreasing the lattice constant, for example, phosphorous, causes defects or the like due to the misfit. The misfit may occur in the order of 10.sup.-2 % or less. Therefore the mismatch associated with electrically insulating materials is decidedly large. Accordingly, whenever factors deteriorating the crystal, for example the contamination with foreign matters are removed, the grown layer has high strains developed therein upon forming the lattice. This greatly affects noise, carrier mobility, lifetime of carriers, etc. relating to the grown layer.
Similary, with gallium arsenide (GaAs) grown on a spinel (MgAl.sub.2 O.sub.4), a misfit or mismatch ranges from 4 to 5% and therefore is further increased to additionally deteriorate the crystallographic properties of the grown gallium arsenide as compared with a silicon crystal grown thereon. Also it is well known that sapphire can be used as electrically insulating crystals but the predetermined relationship exists between the same and, for example, silicon grown thereon in terms of the crystal face and crystallographic direction. For example, a silicon crystal grown on sapphire has a crystal face (111) grown on a crystal face (0001) of the sapphire and a crystallographic direction [110] parallel to that represented by [1120] of the latter. Further, silicon atoms are arranged at intervals of 3.85 A in the direction [110], while aluminum atoms are arranged at intervals of 4.75 A in the direction [1120]. Assuming that the silicon atoms in the silicon crystal correspond to the aluminum atoms in the sapphire, the resulting mismatch may be as large as in the order of 20%. Also in this case, the silicon crystal has a crystal face (100) grown on a crystal face (1012) of the sapphire and a crystallographic direction [100] parallel to both directions [1210] and [1011] thereof and a crystallographic direction [110] parallel to a direction [2201] of the sapphire. Further the silicon crystal has interatomic intervals of 5.43 and 3.82 A in the crystallographic directions [100] and [110], respectively. On the other hand, the sapphire has interatomic intervals for alluminum of 4.75, 5.2 and 3.52 A in the crystallographic directions [1210], [1011] and [2201] of the same and therefore mismatches in those directions reach 12%, 4% and 8% respectively. In this way, a silicon crystal grown on sapphire has different mismatches in different crystallographic directions and the magnitudes thereof are very large.
The misfit due to a difference in lattice constant between dissimilar lattices might have been previously researched but it has not been heretofore considered that a layer of semiconductive material is grown on a substrate formed of an electrically insulating material while the difference in lattice constant between the semiconductor layer and the insulating crystal undergoes a decrease. This is because a heterojunction exists at the interface between the layer and crystal. However it is to be understood that the single crystal disposed on such an insulating member should be necessarily a perfect crystal of a crystal free from any defect. Such single crystals will be demanded in the future.